In a typical cellular radio communication system, also referred to as a wireless communication system, user equipments, also known as mobile terminals and/or wireless terminals communicate via a Radio Access Network (RAN) to one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNB”, “eNodeB”, “NodeB” or “B node” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within the range of the base stations.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter or sending node and the receiver or receiving node are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The Long Term Evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. LTE-Advanced, i e 3GPP Release-10, enables support of eight-layer spatial multiplexing with possible channel dependent preceding. Such spatial multiplexing is aimed for high data rates in favorable channel conditions. An illustration of preceded spatial multiplexing is provided in FIG. 1.
In preceded spatial multiplexing, the information carrying symbol vector s is multiplied by an NT×r precoder matrix WNT×r, which serves to distribute the transmit energy in a subspace of the NT dimensional vector space, which corresponds to NT antenna ports of the sending node. The r symbols in s each are part of a symbol stream, a so-called layer, and r is referred to as the rank or transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time-frequency resource element (TFRE).
The number of layers or rank, r, is typically adapted to suit the current channel properties. Furthermore, the precoder matrix is often selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which for a given rank specifies a unique precoder matrix in the codebook. If the precoder matrix is confined to have orthonormal columns, then the design of the codebook of precoder matrices corresponds to a Grassmannian subspace packing problem.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, and Discrete Fourier Transform (DFT) preceded OFDM in the uplink, and hence the received NR×1 vector yn of data on the TFRE indexed n, is modeled byyn=HnWNT×rsn+en  (1)where n denotes a transmission occasion in time and frequency, en is a noise plus interference vector modeled as realizations of a random process. The precoder, or precoder matrix, for rank r, WNT×r, can be a wideband precoder, which is either constant over frequency, or frequency selective.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent preceding. When based on User Equipment (UE) feedback, this is commonly referred to as closed-loop preceding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
In closed-loop precoding, the UE transmits, based on channel measurements in the forward link, i e the downlink, recommendations to the eNodeB or base station of a suitable precoder to use. A single precoder that is supposed to cover a large bandwidth, so called wideband precoding, may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband. This is an example of the more general case of channel state information (CSI) feedback, which also encompasses feeding back other entities or information than precoders to assist the eNodeB or base station in subsequent transmissions to the LIE. Such other information may include channel quality indicators (CQIs) as well as rank indicator (RI).
Signal and channel quality estimation is a fundamental part of a modern wireless system. Noise and interference estimates are used not only in the demodulator, but are also important quantities when estimating, for example, the channel quality indicator (CQI), which is typically used for link adaptation and scheduling decisions on the eNodeB or base station side.
The term en in (1) represents noise and interference in a TFRE and is typically characterized in terms of second order statistics such as variance and correlation. The interference can be estimated in several ways, for example from cell-specific reference symbols (RS) that are present in the time-frequency grid of LTE. Such RS may correspond to the 3GPP Release-8 cell specific RS, transmitted on antenna ports 0-3, as well as the new CSI RS available in 3GPP Release -10.
Estimates or interference and noise can be formed in various ways. Estimates can easily be formed based on TFREs containing cell specific RS since sn and WNT×r are then known and Hn is given by the channel estimator. It is further noted that the interference on TFREs with data, that is scheduled for the UE in question, also can be estimated as soon as the symbols, e g data symbols, sn are detected, as at that moment they can be regarded as known symbols. The latter interference can alternatively also be estimated based on second order statistics of the received signal and the signal yn intended for the UE of interest, thus possibly avoiding needing to decode the transmission before estimating the interference term.
Thus, in a wireless communication system employing multiple-input multiple-output (MIMO) communication where a transmitter or sending node, e g a base station or an eNodeB, having NT antenna ports transmits information carried in a symbol vector a comprising r symbols over a MIMO communication channel to a receiver or receiving node, e g a UE, having NR antenna ports, each of the r symbols in the information carrying symbol vector s is part of one of r symbol streams between the sending node and the receiving node, also called layers, and r is referred to as the rank or transmission rank. The number of layers or rank ,r, is typically adapted to suit the current channel properties and a precoder matrix WNT×r for rank r is chosen. The precoder matrix may be chosen to match the characteristics of the NR×NT MIMO channel matrix Hn. In closed-loop precoding, the receiving node transmits, based on channel measurements in the forward link, i e the downlink, recommendations to the sending node of a suitable precoder matrix to use. In addition, the receiving node may need to estimate and report, for example, the channel quality indicator (CQI), which is typically used for link adaptation and scheduling decisions by the sending node.
In practice, CQIs are rarely perfect and substantial errors might be present which means that the estimated channel quality does not correspond to the actual channel quality seen for the link over which the transmission takes place. The eNodeB or base station can to some extent reduce the detrimental effects of erroneous CQI reporting by means of outer-loop adjustment of the CQI values. By monitoring the acknowledgement/non-acknowledgement (ACK/NACK) signaling of the hybrid automatic repeat request, (ARQ), the eNodeB or base station can detect if the block error rate (BLER), or a related measure, is below or above the target value. Using this information, the eNodeB or base station can decide to use more offensive, i a less robust, modulation and coding scheme (MCS) than recommended by the UE. Alternatively, the eNodeB can decide to use a more defensive, i a more robust, modulation and coding scheme (MCS) than recommended by the UE based on the information about BLER or a related measure. However, it is more difficult for the eNodeB or base station to deviate from recommended rank, because the CQI reports relate directly to the rank. A change in rank therefore renders the information provided by the CQI reports difficult or impossible to utilize—that is, the eNodeB or base station would have severe difficulties knowing which MCS to use on the different data streams if the eNodeB or base station would override the rank recommended by the UE.